Field of the Invention
The present invention relates to implantable medical devices, and more particularly to implantable cardiac pacemakers or pacers that are capable of "pacing" and "sensing" at least one chamber of a heart. Even more particularly, the present invention relates to discharging a coupling capacitor used to couple a pace/sense lead to the cardiac pacemaker so that the pace/sense lead can more quickly be used for "sensing" following delivery of an electrical pulse through the pace/sense lead, i.e., following "pacing."
Generally, a heart stimulator, commonly known as a "pacemaker" or "pacer," uses one or two flexible pace/sense leads having one end (a proximal end) connected to the pacer and the other end (a distal end) connected to electrodes placed in close proximity to the heart. These pace/sense leads are used to stimulate or pace the heart. Also, these pace/sense leads are used to sense the heart activity by picking up electrical signals from the heart.
In order to properly pace or sense, the pacer has to be able to deliver a stimulating pulse to the heart or sense an electrical signal from the heart. This requires that there be an electrical return path for the stimulating pulse or the electrical signal from the heart. If, within a given heart chamber, a unipolar pace/sense lead is used--containing a single conductor--the return path is the conductive body tissues and fluids. The return path is connected to the pacer by connecting the pacer's ground to the pacer's metal enclosure, typically referred to as the pacer "case." The case, in turn, makes contact with the body tissue and/or fluids.
Problematically, the conductive body tissues and fluids are frequently poor conductors. Therefore, a differential voltage may be generated across the return path as the stimulating pulse or electrical signal passes through the body tissues and fluids.
An alternative solution to using a unipolar pace/sense lead in a given heart chamber, or elsewhere within the body, is to use a double lead/electrode, known as a bipolar pace/sense lead. In a bipolar pace/sense lead, a second conductor is generally spiraled over and insulated from a first conductor along the length of the pace/sense lead. At the distal end of the pace/sense lead, one of the conductors is connected to a first electrode, referred to as the "tip" electrode, and the second conductor is connected to a second electrode, referred to as a "ring" electrode. The ring electrode is generally situated 10 to 20 mm from the tip electrode. The tip electrode is typically placed in contact with heart tissue, while the ring electrode is in electrical contact with the blood or other body fluids. Because both body tissue and fluids are conductive, the ring electrode of a bipolar pace/sense lead, in contact with the body fluids, serves as an electrical return for both pacing and sensing. Because the ring electrode is located very close, e.g. 10 to 20 mm from the tip electrode, any differential voltage is eliminated or substantially reduced across the return path.
Thus, pacing or sensing using the pacer case or enclosure as a part of the electrical return path is referred to "unipolar pacing" and "unipolar sensing," whereas pacing or sensing using the pace/sense lead ring electrode and associated pace/sense lead conductor as the electrical return path is referred to as bipolar pacing" and "bipolar sensing."
Typically, a pacer delivers a stimulating current pulse by switchably connecting the electrode tip, through a coupling capacitor, to the negative terminal of a storage capacitor, the positive terminal of the storage capacitor being coupled to the pacer's ground. The voltage stored on this storage capacitor has previously been adjusted or amplified to the desired magnitude by a charging circuit within the pacer. The coupling capacitor is required to prevent DC current from flowing through the tip electrode/body interface. The return path for the pacing pulse is provided by grounding the case or ring electrode, depending on whether unipolar or bipolar pacing is being performed. After delivering the pulse, the coupling capacitor remains charged with a positive charge on its tip electrode side (distal side). The pacer side of the coupling capacitor (proximal side) likewise remains charged. Problematically, this pacer-side charge causes distortions in the sensing of cardiac activity and therefore prevents accurate sensing of cardiac activity through the pace/sense lead. Therefore the pacer-side charge must be removed before such sensing can be accurately achieved.
Heretofore, the pacer-side charge has been removed by connecting it through a discharging switch, and possibly a discharging resistor, to the reference potential. See, e.g. U.S. Pat. Nos. 4,114,627; 4,373,531; 4,858,610; 4,991,583; and 5,170,806. Problematically, this approach to discharging the coupling capacitor provides only limited control over the coupling capacitor's discharge rate, and generally requires the use of at least one resistor, which is problematic if the pacer is constructed on an integrated circuit due to the relatively large space requirement for fabricating a resistor.
Thus, improvements are needed in the discharge circuitry used with a coupling capacitor of a cardiac pacemaker or other implantable stimulator so that the pace/sense lead can more quickly be used for "sensing" following delivery of an electrical pulse, i.e., following "pacing." The present invention advantageously addresses the above and other needs.